Rotary potentiometer



Jan. 8, 1963 w. F. O'SHEA 3,072,872

ROTARY FOTENTIOMETER Filed Feb. 15, 1961 2 Sheets-Sheet 1 INVENTOR.

muurm F- O$HEH Jan. 8, 1963 w. F. O'SHEA 3,072,872

ROTARY POTENTIOMETER Filed Feb. 15, 1961 2 Sheets-Shetz Milli Lai.

Hill 28 I 39 INVENTOR.

Nun/7MP. o'sHEH United States Patent Ofifice Patented Jan. 8, 1963 Thisinvention relates to electrical resistance devices, and moreparticularly relates to devices of the variable resistance type knowngenerally as potentiometers.

The potentiometer according to the invention embodies a number of novelfeatures which cooperate to provide an electrically reliable,mechanically rugged unit of relatively low cost which may be easily andquickly assembled by comparatively unskilled personnel. One novel aspectof the potentiometer according to the invention resides in l aresistance element which is disposed within the poten-.

tiometer housing and held in mechanically stabilized position by a novelinterlocking arrangement which does not require rivets, cementing orother permanent type securement. One advantage of this constructionresides in the great simplification of assembly, and also optionallypermits the simple and rapid replacement of the resistance element inthe field. Indeed, under emergency conditions, temporary emergencyrepairs may even be effected in the field by virtue of the potentiometerconstruction which permits easy removal of the resistance element fromthe composite unit without physical demounting of the potentiometercasework. Additional features of the novel potentiometer constructioninclude a potentiometer caseforming treatment which efiectivel'yincreases the heat dissipation rating of the unit, a novel rotorstructure and electrical connection of the movable rotor arm to anexternal terminal, and a composite assembly comprising a minimum numberof parts. Accordingly, it is a primary object of this invention toprovide a novel potentiometer incorporating the aforedescribed features.

Another object of this invention is to provide a novel potentiometerwhich incorporates a one-piece molded plastic liner disposed within andsecured to the potentiometer cover shell.

Still another object of this invention is to provide a novelpotentiometer structure including a quickly demountable resistanceelement carried by a plastic former of a resilient nature whichmechanically interfits with the plastic cover shell liner.

Yet another object of this invention is to provide a novel potentiometerwherein the movable arm of the potentiometer is electrically coupled toan external terminal which is carried by the cover shell backplate, andwherein securement of the backplate to the cover shell furthermechanically stabilizes the electrical resistance element.

The foregoing and other objects of the invention will become apparentfrom a reading of the following specification in conjunction with anexamination of the appended drawings, wherein:

FIGURE 1 is a side elevational view of the novel potentiometer accordingto the invention as would be seen when facing the three contactterminals projecting externally of the unit;

FIGURE 2 is a top plan view of the potentiometer shown in side elevationin FIGURE 1;

FIGURE 3 is a perspective exploded view of the poten tiometer with thebackplate, rotor and shaft, and resistance element assembly allphysically separated from the potentiometer cover shell and separatedone from another;

FIGURE 4 is a vertical sectional view through the assembledpotentiometer as would be seen when viewed along the lines 44 of FIGURE1;

FIGURE 5 is an inverted plan view of the interior of the assembledpotentiometer as would be seen when viewed along the lines 5-5 of FIGURE4, the rotor assembly being also illustrated in phantom in one extremeposition of rotation;

FIGURE 6 is a fragmentary vertical section as would be seen when viewedalong the lines 6 -6 of FIGURE 4; and

FIGURE 7 is a fragmented view as would be seen when viewed along thelines 77' of FIGURE 6 and showing a terminal affixed to one end of theresistance element with the resistance element disposed within a channelportion of the plastic shell liner.

In the several figures, reference characters.

Referring first to FIGURES l, 2 and 3, the gross aspects of thepotentiometer according to the invention as disclosed therein comprise acylindrical metal cover shell 10 closed at one end 10a, the closed end1011 being provided with a central depression or well 12 through whichaxially projects an externally threaded hollow bushing 13 adapted tohave passed axially therethrough and partially disposed therewithin thesegmented rotor shaft 14. The rotor shaft 14 is divided into a shorterupper segment 14a and a longer lower segment 1419 by an annular groove15 within which is disposed an O-ring 16. When the shaft 14 has beenslid through the bushing 13, the 0 ring 16, when in place, preventsretraction of the shaft backward through the bushing 13, as best seen inthe showing of FIGURE 4. The cylindrical side wall portion of the covershell 10 is deep-scored or grooved as at 11 to provide a greatlyincreasedsurface area to the cover shell by means of which more rapidand effective cooling thereof may be obtained.

The open end of the cover shell 10 is provided with a plurality oflocking tabs 17 which extend in axial continuation of the annular sidewall of the cylindrical cover shell 10, and which in the assembled unitsare disposed within the correspondingly located notches 18 cut into theperiphery of the cover shell insulating material backplate 19, beingthereafter turned radially inward into close overlying engagement withthe outer surface of the backplate to secure the latter in place.Tab-locked or otherwise secured to the backplate 19 is a metallic rotorcontact plate 20 having an external terminal part 21 integrally formedtherewith and extending therefrom radially outward beyond the edge ofthe backplate 19. Centrally punched through the contact plate 20 so asto be disposed on the axis of the cylindrical bushing 13 is a circularsocket hole 22. Disposed in peripheral engagement with the edge of thecircular socket hole 22 is a spring biased metallic contact cup 23 notvisible in FIGURES 1, 2 or 3, but which is shown in FIGURES 4 and 5 asconstituting a portion of the total rotor contact assembly. The contactcup 23 is symmetrically centered on the axis of the rotor shaft 14 sothat it properly aligns with the socket hole 22 on the rotor contactplate 20, this alignment being best seen in the showing of FIGURE 4.

As best seen in FIGURE 3, the resistance element itself comprises a fiatstrip of plastic insulating material configure in the form of a splitcylindrical annulus 24, the ends 25 of the annulus being straight-linetapered from top to bottom so that when sprung together the bottompoints 26 would contact while leaving a gap at the upper edges 27 tothereby define a triangular opening between the ends. This tapered endconstruction will be seen sub sequently to provide a camming action witha portion of the plastic liner for the cover shell effective tomechanically stabilize the resistance element in proper position withinthe interior of the unit. Wound about the annulus 24 is a continuouslength of resistance wire 28, the op posite ends of which terminateproximate the ends of the annulus 24, being there secured in place bythe clamping action of the end terminals 29. As best seen in FIGURE 7,each of the terminals 29 includes a U-shaped portion like elements aredenoted by like 3d disposed closely about the resistance wire 28 and theannulus 24 so as to clamp the latter therewithin, fixed securement beingprovided by the rivets 31. Turned outward from each of the U-shapedterminal portions 30 is an external terminal piece 32 by means of whichexternal circuit connections to opposite ends of the resistance elementmay be made.

Staked or otherwise secured to the'bottom of the shaft 14 in a planeoriented orthogonally to the axis of the shaft is the rotor supportinsulator disc 33 which carries the metallic rotor contact assembly, theresistance element engaging finger 34 of this contact assembly beingvisible in the perspective view of FLGURE 3. As best seen in FiG- URES 4and 5, the rotor contact assembly is secured to the support disc 33 by apair of rivets 35 so positioned relative to the support disc 33 that thecontact finger 34 centrally underlies the rotor stop wedge 36, whichlatter extends laterally from a portion of the periphery of the 'rotorsupport disc 33 and provides a stop action for the rotor contact 34 atthe extreme positions of rotation by engaging the sides of the wedgeportion 41 of the plastic liner 3? to be now described. The separateelements shown in exploded relationship in FIGURE 3 are assembled intothe composite unit of FiGURE 1, the assembled details of which are shownprincipally in the illustrations of FIGURES 4 and to which particularreference should now be made.

As best seen from FIGURES 4 and 5, the cover shell isis completely linedwith a molded plastic liner 37 which conforms to the inside surface ofthe cover shell. The liner 37 includes a centrally apertured basesurface 38 which covers the inside surface of the cover shell endclosure lllu', a peripherally extending side wall portion 39 which linesthe inside surface of the cylindrical scored portion of the cover shell16, an annular wall 40 projecting from the base surface 38 concentricwith the peripheral wall 39 but of smaller radius, and a triangularwedge portion 41 extending between the annular wall 40 and theperipheral wall 39 in the region between the recesses 42 of the covershell side wall through which the terminals 32 of the resistance elementassembly outwardly project. The liner peripheral wall 39 is recessedtoward the base surface 3% to provide a shoulder 43 which extendssubstantially completely about the cylindrical wall periphery of theliner 37 but which terminates at the sides of the wedge portion 41, thisbeing best seen in the showing of FIGURE 5. The peripheral wall 39 ofthe liner 37 is additionally downward recessed to the level of theshoulder 43 in the region of the cover shell terminal recesses 42 sothat the terminals 32 may pass outward therethrough. As most clearlyseen in FIGURES 3 and 7, the liner shoulder 43 extends beyond the edgeof the cover shell recesses 42 so that the projecting terminals 32 areprevented from physically engaging the cover shell. The liner peripheralwall 39 similarly prevents the terminals 32 from contacting the covershell annular side wall portions disposed laterally thereof.

As best seen in the view of FIGURE 5, the side walls 44 of the wedgeportion 41 taper outwardly as they progress from the wedge top surfacetoward the base surface 38 of the plastic liner 37 so that the taperedends 25 of the resistance element annular former 24 are cammed apartfrom one another as the entire resistance element is slipped into theplastic liner to thereby radially expand the annular former 24 and causeit to press outward against the inside surface of the liner peripheralwall 39 between the liner shouldered region 43 and base wall 38. Thelength of the resistance element annulus 24 is chosen so that when theupper edges 27 thereof substantially contact the base surface 33 of, theliner 37, the tapered ends 25 of the former 24 are disposedsubstantially at the intersection bctweeen the wedge side walls 44 andthe shou de ed portion 43 of the liner peripheral wall 3-9, and theresistance element terminals 32 side-abut the recessed edges of theperipheral wall 39 located at the cover shell terminal recesses 42. Theentire resistance element assembly is thus stabilized in a radialdirection, being unable to either increase or decrease its radiuswithout first being moved outward toward the cover shell back-plate 19.Such motion toward the cover shell backplate 19 is prevented by the tab45 projecting in continuation of the U-shaped terminal portion 343 ofeach end terminal 29. These tabs are of such length that the endsthereof are just engaged by the inside surface of the cover shellbackpla-te 1? when the latter is placed in position, thus preventingmotion of the resistance element in the direction of the cover shellbackplate 19 and completely mechanically stabilizing the same.

As also best seen in FIGURES 4 and 5, the rotor contact assembly isstamped out of a piece of resilient spring metal to provide a generallycircularly shaped disc form from which laterally projects the contactfinger 34, a til-shaped region being removed from the main body of thespring metal to provide a pair of arcuate arms 46 which connect thecontact finger 34 to the securement reigon 47 where the contactstructure is riveted to the support disc 33 by the rivets 35 aspreviously described.

. re arcua-te arms 46 impart resiliency to the structure to allow thecontact finger 34 to be resiliently displaced from the plane of thesecurement region 47 in the manner seen in FIGURE 4. The relatively longsweep of the arms 46 provides for a gradual displacement thereof fromthe undersurface of the rotor support disc 33 and thereby completelyeliminates any localized flexing stresses. The central section of thecontact structure which carries the contact cup 23 is also bent up outof the plane of the main securement region 47 in the manner indicated at48. The bend imparted at 48 and the depth of the contact cup 23 aresufiicient so that the cup engages the socket hole 22 of the rot-orcontact plate 20 when the cover shell backplate 19 is secured to thecover shell 10. The terminal 21 extending from the rotor contact plate20 thus lies fla-twise on the upper surface of the wedge portion 41 andis thereby supportedly clamped between the wedge and the backplate 19.

Assembly of the unit is achieved in the following manner. First, themolded liner 37 is placed inside the cover shell 10 and properlyoriented as best seen in FIGURES 3 and 5. Next, the bushing 13 isprojected through the central aperture in the plastic liner and thecorresponding hole in the cover shell well 12, so that the bushingshoulder 49 clamps the base surface 38 of the liner 37 against theinside surface of the cover shell Well 12. Then, the outwardly facingsurface 59 of the bushing 1.3 which lies above and somewhat radiallyinward of the bushing shoulder 49 is radially outwardly staked orcrimped as at 51 to overlie the periphery of the cover shell well 12external surface to simultaneously fixedly secure the plastic liner 37and bushing 13 to the cover shell 10. The resistance element is thenplaced into position within the liner and pressed firmly downward intoseating position. Thereafter, the rotor is assembled to the unit, byprojecting the shaft 14 through the bushing 13 and slipping the 0 ring16 down into locking position with the annular groove 15 which separatesthe segments 14a and 14b of the rotor shaft 14. Finally, the backplate19 is fitted onto the back of the cover shell with the socket hole 22 ofthe contact plate 20 seated onto the contact cup 23, and the locking tab17 are bent radially inward over the outer surface of the backplate 19to secure the entire assembly together.

From the foregoing description, and particularly the showing of FIGURE4, it will be appreciated that assembling of the backplate 19 to thecover shell 10 imparts an upward bias to the entire rotor and shaftassembly through the resilient bend 48 which joins the contact cup 23 tothe securement region 47 of the rotor contact to thereby eifect apositive contact between the socket hole 22 of the contact plate 20 andthe contact cup 23 of the rotor contact assembly. The upward biasimparted to the rotor and shaft assembly imparts a bias to the arcuatearms 46 to thereby insure positive engagement between the rotor assemblycontact finger 34 and the resistance wire 28 of the resistive unit.Additionally, the O ring groove 15 on the rotor shaft 14 is so locatedthat axial pressure applied to the end of the shaft 14 cannot cause suchdisplacement of the rotor assembly so as to break the contact betweenthe resistance element and the contact finger 34 of the rotorcontactassembly.

Having now described my invention in connection with one particularlyillustrated embodiment thereof, it will be apparent that modificationsand variations may occur from time to time to those persons normallyskilled in the art without departing from the essential scope or spiritof my invention, and accordingly, it is intended to claim the samebroadly as well as specifically as indicated by the appended claims.

What is claimed as new and useful is:

1. A rotary variable resistance unit comprising in combination, a hollowcylindrical housing closed at one end and open at the other end, amolded insulation liner completely lining the inside surface of thehousing, an annular resistance element assembly disposed within thehousing and separated at all points therefrom by the intervening liner,said liner and resistance element assembly having complementally shapedengageable parts effective when engaged to radially expand theresistance element assembly into engagement with the liner substantiallycompletely about the outer periphery of the annular resistance elementassembly and retain the latter in such radially expanded condition, arotor mounted contact assembly including a movable contact fingeradapted for wiping contact with the resistance element at any desiredpoint between the ends thereof, contact means for electricallyconnecting the contact assembly to a terminal projecting externally ofthe housing, and terminal means electrically connected to the resistanceelement and projecting externally of the housing.

2. The variable resistance unit according to claim 1 wherein thecomplementally shaped engageable parts of said liner and resistanceelement assembly comprise respectively, a liner wedge portion havingside walls extending divergingly radially inward from the liner sidewall toward the rotor, said wedge portion side walls also extending indiverging fashion from the housing open end to the liner base walllining the housing closed end, the resistanceelement assembly includingan annular former longitudinally split to provide a pair of ends havingdivergingly tapered facing edges, the tapered edges of the annularformer being engageable with the tapered side walls of the liner wedgeand being cammed apart to radially expand the annular former as thelatter is moved into the housing from the open end toward the closed endthereof.

3. The variable resistance unit according to claim 1 wherein the rotormounted contact assembly further includes a contact cup electricallycoupled to the said contact finger through a pair of resilient arcuatearms, said contact cup having a curved bottom surface orientedsubstantially symmetrically to the axis of rotor rotation and rotatablewith the rotor about that axis, and said contact cup resiliently engagesthe said contact means for electrically connecting the contact assemblyto terminal projecting externally of the housing.

4. The variable resistance unit according to claim 1 wherein the rotormounted contact assembly further includes a contact cup electricallycoupled to the said contact finger through a pair of resilient arcuatearms, said contact cup having a curved bottom surface orientedsubstantially symmetrically to the axis of rotor rotation and rotatablewith the rotor about that axis, and said contact cup resiliently engagesthe said contact means for electrically connecting the contact assemblyto a terminal projecting externally of the housing, said contact meanscomprising a socketed contact plate mounted upon the inside surface of abackplate secured to and closing the open end of the said housing, saidcontact cup being seated in the contact plate socket and the saidterminal being an extension of the said contact plate.

5. The variable resistance unit according to claim 1 wherein thecomplementally shaped engageable parts of said liner and resistanceelement assembly comprise respectively, a liner wedge portion havingside walls extending divergingly radially inward from the liner sidewall toward the rotor, said wedge portion side walls also extending indiverging fashion from the housing open end to the liner base walllining the housing closed end, the resistance element assembly includingan annular former longitudinally split to provide a pair of ends havingdiverging tapered facing edges, the tapered edges of the annular formerbeing engageable with the tapered side walls of the liner wedge andbeing cammed apart to radially expand the annular former as the latteris moved into the housing from the open end toward the closed endthereof, and wherein the rotor upon which the said contact assembly ismounted includes a rotor stop projection effective to prevent more thana predetermined range of rotor rotation by abutment with the liner wedgeportion side walls.

6. A rotary variable resistance unit comprising in combination, a hollowcylindrical metal housing open at one end and having a centrallyapertured closure wall at the other end, a molded insulation linerdisposed close fittingly within the housing for completely lining theinside surface thereof and being conformingly apertured to the centralaperture of the housing closure wall and in alignment therewith, ashouldered bushing projected through the aligned apertures of the linerand housing so that the bushing shoulder presses the liner against thehousing closure wall, and means securing the bushing in position tothereby fixedly clamp together the housing and liner and bushing, saidbushing being centrally bored to provide therethrough a cylindricalpassage coaxial with the cylindrical axis of the housing, an annularresistance element assembly disposed within the housing and separated atall points therefrom by the intervening liner, a rotatable shaft closefittingly disposed within the bushing passage and extending therebeyondinto the housing interior, a rotor secured to the shaft for rotationtherewith within the housing interior and carrying a contact assemblyfor making wiping contact with the resistance element, external terminalmeans electrically connected to the said contact assembly and to atleast one fixed point of the resistance element, and cover means forclosing the open end of the said housing. I

7. A rotary variable resistance unit comprising in combination, a hollowcylindrical metal housing open at one end and having a centrallyapertured closure wall at the other end, a molded insulation linerdisposed close fittingly within the housing for completely lining theinside surface thereof and being conformingly apertured to the centralaperture of the housing closure wall and in alignment therewith, ashouldered bushing projected through the aligned apertures of the linerand housing so that the bushing shoulder presses the liner against thehousing closure wall, and means securing the bushing in position tothereby fixedly clamp together the housing and liner and bushing, saidbushing being centrally bored to provide there through a cylindricalpassage coaxial with the cylindrical axis of the housing, an annularresistance element assembly disposed within the housing and separated atall points therefrom by the intervening liner, said liner and resistanceelement assembly having complementally shaped engageable parts effectivewhen engaged to radially expand the resistance element assembly intoengagement with the liner substantially completely about the outerperiphery of the annular resistance element assembly and retain thelatter in such radially expanded condition, a rotatable shaft closefittingly disposed within the bushing passage and extending therebeyondinto the housing interior, a rotor secured to the shaft for rotationtherewith within the housing interior and carrying a contact assemblyfor making wiping contact with the resistance element, external terminalmeans electrically connected to the said contact assembly and to atleast one fixed point of the resistance element, and cover means forclosing the open end of the said housing.

8. The variable resistance unit accordance to claim 7 wherein thecomplementally shaped engageable parts of said liner and resistanceelement assembly comprise respectively, a liner wedge portion havingside walls extending divergingly radially inward from the liner sidewall toward the rotor, said wedge portion side walls also extending indiverging fashion from the housing open end to the liner base walllining the housing closed end, the resistance element assembly includingan anular former longitudinally split to provide a pair of ends havingd1- vergingly tapered facing edges, the tapered edges of the annularformer being engageable with the tapered side walls of the liner wedgeand being cammed apart to radially expand the annular former as thelatter is moved into the housing from the open end toward the closed endthereof.

9. The variable resistance unit according to claim 7 wherein thecomplementally shaped engageable parts of said liner and resistanceelement assembly comprise respectively, a liner wedge portion havingside walls extending divergingly radially inward from the liner sidewall toward the rotor, said wedge portion side wall-s also extending indiverging fashion from the housing open end to the liner base walllining the housing closed end, the resistance element assembly includingan annular former longitudinally split to provide a pair of ends havingdivergingly tapered facing edges, the tapered edges of the annularformer being engageable with the tapered side walls of the liner wedgeand being cammed apart to radially expand the annular former as thelatter is moved into the housing from the open end toward the closed endthereof, and wherein the rotor upon which the said contact assembly ismounted includes a rotor stop projection effective to prevent more thana predetermined range of rotor rotation by abutment with the liner wedgeportion side walls.

10. A rotary variable resistance unit comprising in combination, ahollow cylindrical housing closed at one end and open at the other end,a molded insulation liner completely lining the inside surface of thehousing, an annular resistance element assembly disposed within thehousing and separated at all points therefrom by the intervening liner,said liner and resistance element assembly having complementally shapedengageable parts effective when engaged to radially expand theresistance element assembly into engagement with the liner susbtantiallycompletely about the outer periphery of the annular resistance elementassembly and retain the latter in such radially expanded condition, arotor mounted contact assembly including a movable contact fingeradapted for wiping contact with the resistance element at any desiredpoint between the ends thereof, contact means for electricallyconnecting the contact assembly to a terminal projecting externally ofthe housing, terminal means electrically connected to the resistanceelement and projecting externally of the housing, and a housingbackplate secured to and covering the open end of the said housing, saidresistance element assembly also including means projecting therefromtoward and proximate to the inside surface of said backplate wherebysaid resistance element assembly is prevented from shifting axiallywithin the cylindrical housing when said backplate is secured inposition.

11. .A rota y variable resi tance unit comprising in combination, ahollow cylindrical housing closed at one end and open at the other end,the exterior surface of the housing cylindrical side walls being scoredwith a plurality of grooves to effectively increase the heat radiatingsurface of the housing, a molded insulation liner completely lining theinside surface of the housing, an annular resistance element assemblydisposed within the housing and separated at all points therefrom by theintervening liner, said liner and resistance element assembly havingcomplementally shaped engageable parts effective when enga ed toradially expand the resistance element assembly into engagement with theliner substantially completely about the outer periphery of the annularresistance element assembly and retain the latter in such radiallyexpanded condition, a rotor mounted contact assembly including a movablecontact finger adapted for wiping contact with the resistance element atany desired point between the ends thereof, contact means forelectrically connecting the contact assembly to a terminal projectingexternally of the housing, and terminal means e1ectrically connected tothe resistance element and projecting externally of the housing.

12. A rotary variable resistance unit comprising in combination, ahollow cylindrical housing closed at one end and open at the other end,a molded insulation liner completely lining the inside surface of thehousing, an annular resistance element assembly disposed within thehousing and separated at all points therefrom by the inter.- veningliner, said liner and resistance element assembly having complementallyshaped engageable parts effec! tive when engaged to radially expand theresistance ele! ment assembly into engagement with the liner substantially completely about the outer periphery of the annular resistanceelement assembly and retain the latter in such radially expandedcondition, a rotor mounted contact assembly including a movable contactfinger adapted for wiping contact with the resistance element at anydesired point between the ends thereof, contact means for electricallyconnecting the contact assembly to a terminal projecting externally ofthe housing, terminal means electrically connected to the resistanceelement and projecting externally of the housing, the complementallyshaped engageable part of said liner engageable with said resistanceelement assembly comprising a liner wedge portion hav.- ing side wallsextending divergingly radially inward from the liner side wall towardthe rotor, said wedge portion side walls also extending in divergingfashion from the housing open end to the liner base wall lining thehousing closed end, the complementally shaped part of said resistanceelement assembly engageable with said liner including an annular formerlongitudinally split to provide a pair of ends having divergingl-ytapered facing edges, the tapered edges of the annular former beingengageable with the tapered side walls of the liner wedge and beingcammed apart to radially expand the annular former as the latter ismoved into the housing from the open end toward the closed end thereof,and a housing backplate secured to and covering the open end of the saidhousing, said resistance element assembly also including meansprojecting therefrom toward and proximate to the inside surface of saidbackplate whereby said resistance element assembly is prevented fromshifting outward toward the housing open end when said backplate issecured in position.

References fi ted in the file of this patent UNITED STATES PATENTS1,751,231 Davis Mar. 18, 1930 1,876,921 Hall Sept. 13, 1932 2,177,290Schellenger Oct. 24, 1939 2,266,l88 Foley et al Dec. 16, 1941

1. A ROTARY VARIABLE RESISTANCE UNIT COMPRISING IN COMBINATION, A HOLLOWCYLINDRICAL HOUSING CLOSED AT ONE END AND OPEN AT THE OTHER END, AMOLDED INCULATION LINER COMPLETELY LINING THE INSIDE SURFACE OF THEHOUSING, AN ANNULAR RESISTANCE ELEMENT ASSEMBLY DISPOSED WITHIN THEHOUSING AND SEPARATED AT ALL POINTS THEREFROM BY THE INTERVENING LINER,SAID LINER AND RESISTANCE ELEMENT ASSEMBLY HAVING COMPLEMENTALLY SHAPEDENGAGEABLE PARTS EFFECTIVE WHEN ENGAGED TO RADIALLY EXPAND THERESISTANCE ELEMENT ASSEMBLY INTO ENGAGEMENT WITH THE LINER SUBSTANTIALLYCOMPLETELY ABOUT THE OUTER PERIPHERY OF THE ANNULAR RESISTANCE ELEMENTASSEMBLY AND RETAIN THE LATTER IN SUCH RADIALLY EXPANDED CONDITION, AROTOR MOUNTED CONTACT ASSEMBLY INCLUDING A MOVABLE CONTACT FINGERADAPTED FOR WIPING CONTACT WITH THE RESISTANCE ELEMENT AT ANY DESIREDPOINT BETWEEN THE ENDS THEREOF, CONTACT MEANS FOR ELECTRICALLYCONNECTING THE CONTACT ASSEMBLY TO A TERMINAL PROJECTING EXTERNALLY OFTHE HOUSING, AND TERMINAL MEANS ELECTRICALLY CONNECTED TO THE RESISTANCEELEMENT AND PROJECTING EXTERNALLY OF THE HOUSING.